Fibronectin is a major cell surface and extracellular matrix glycoprotein involved in cell adhesion and migration. Synthetic peptide inhibitors derived from its sequence were examined for effects on a variety of adhesion systems. The pentapeptide Gly- Arg-Gly-Asp-Ser (GRGDS) was the most effective peptide against fibronectin-mediated adhesion, and a related sequence was active for vitronectin. Such peptides had little activity on laminin, collagen, or cell-cell adhesion, indicating specificity. An artificial inverted peptide identified a possible common denominator of function in four of these five systems. GRGDS inhibited experimental metastasis of B16 melanoma cells in mice as measured by pulmonary colonization, and the relative activities of analogues closely matched those for inhibition of cell adhesion in vitro. Peptide treatment also substantially prolonged survival of these animals. Several strategies to increase peptide effectiveness in vitro and in vivo are under investigation. This pentapeptide adhesive recognition sequence also appears to function in Drosophila gastrulation and in vertebrate somite formation; somite precursor cells showed induced cell-cell adhesion. An additional region required for full-affinity binding to the cell surface (50- to 100-fold augmentation) is being defined by recombinant DNA expression studies. Large fibronectin fragments expressed in E. coli as lambda gt11 fusion proteins retained greater than 80% adhesive activity; eukaryotic post- translational modifications were thus not needed. Deletion mutagenesis experiments show that the second site is at least 20kD away from the GRGDS site. A separate class of novel, cell- type specific binding sites were discovered elsewhere in fibronectin at sites regulated by alternative splicing. A synthetic peptide from one site was only 2.4-fold less active than intact fibronectin. The critical amino acid sequences and the biological functions of these novel regions in cell migration and metastasis will be determined.